Description:The Roque Nublo (meaning clouded rock) once revered to by the ancient Guanche population and today visited by thousands of tourists each year is the emblematic symbol of Gran Canaria.
The 80 metre tall volcanic monolith (centre of image) stands at 1,813 meters above sea level and was formed as a result of extensive erosion and is all but very little that remains of the once mighty Roque Nublo stratovolcano which stood in the center of Gran Canaria between approximately 4.6 and 3.0 million years ago and who’s central crater was located close to where the Roque Nublo stands today.
The rock formations of which the Roque Nublo is composed were deposited at some time between 3.9 and 3.0 million years ago, during a period when explosive vulcanian-phreatomagmatic eruptions dominated the Roque Nublo volcano’s activity.
During this period the highly explosive eruptions emitted a series of dense, water vapor and lithic rich, relatively low temperature pyroclastic flows, resulting in a rather unusual type of non-welded ignimbrite (also termed block and ash flow) in which the juvenile components (matrix composed of trachytic to phonolitic pumice fragments and ash) were largely altered into zeolites and subordinate smectites. The alteration took place in situ and immediately after the deposition of the deposits when they were still hot, by the reaction between the vitric juvenile components and the condensed water vapour, which diagenetically cemented the pumice rich block and ash flow.
The combined effect of the magma-water interaction and the high content of lithic fragments is sufficient to explain the characteristic low emplacement temperature of the Roque Nublo ignimbrites (< 500ºC) and the fact that the eruption columns rapidly became too dense to be sustained as vertical eruption columns and were transformed into tephra fountains which fed high-density pyroclastic flows.
These heterogeneous ignimbrites containing varying proportions of crystals, vesiculated pumice, ash and up to 55% Lithics (rock fragments) were to form massive layered sheets of breccia-type pyroclastic deposits known as the Roque Nublo breccia, of up to 60 metres thick in places and extending up to 25 km in length as they were channeled down a number of prehistoric canyons, towards the coastline, of which some flows were transformed into lahars. Many of the breccia sheets are also interlayered with lava flows, composed of basanites, alkaline basalts, tephrites, trachybasalts (hawaiites), basaltic trachyandesites (benmoreite), phonotephrites, tephri-phonolites and phonolites.
As the volcano reached maturity and as the eruptions continued piling up layer upon layer of Ignimbrites, lavas and other fallout deposits, the volcano was to build a high asymmetric cone whose southern flank was much steeper than the northern and western flanks, steep flanks that were to become unstable and begin to suffer from periodic gravitational collapses not only to the south but also to the north and west. These collapses perhaps also associated with the explosive eruptions.
Finally, towards 3 million years (the exact date is uncertain) as the volcano reached the peak of its evolution and having reached an estimated minimum altitude of between 2500 and 3000 metres, a catastrophic edifice failure occurred and the entire southern flank of the volcano collapsed in a giant debris avalanche, much of which was channeled down the area of today’s Barranco de Arguineguin, this event resulting in the formation of an enormous horseshoe shaped crater. Whether this giant and final flank collapse was triggered by an eruption, by seismic movements resulting from intrusions or simply by the collapse of its very steep flanks, is however unclear, certainly there is no evidence of a massive explosive eruption as had formed the Tejeda caldera in the Miocene.
Coinciding with this event, the volcanic activity was waning, this period being marked by the intrusion of a number of phonolitic domes with associated fumarole activity around the periphery of the volcano until around 2.7 million years when activity ceased.
Following this, whatever remained of the volcano’s central cone has been largely erased by 3 million years of erosion on its southern side and mostly buried by post Roque Nublo lavas to the north and east. It is during this time that the Roque Nublo was to take its shape as it is seen today, as have also many other features belonging to the Roque Nublo volcano’s time period, all over the island, such as the Risco Blanco, the Roques de Tenteniguada, the Pico de Las Nieves, the Roque Bentaiga, Mesa de Junquillo, the numerous breccia sheets and avalanche debris in the south, lava flows and of course the core of the volcano itself between La Culata, Ayacata and Presa de los Hornos where many dikes, areas of hydrothermal alteration and plutonic rocks have been exposed, plentiful evidence for scientists to have been able to reconstruct this volcano.
The Roque Nublo monolith once formed part of the uppermost layer of a cerca 400 metre thick, series of breccia sheets, in part destroyed during the collapse of the southern flank. Some of these avalanche debris form giant dislodged blocks, which are very evident between Ayacata and Cruz Grande. The Roque Nublo being the eroded remains of one such dislodged and broken block of breccia sheet. The rock is composed of tephritic and phonolitic lithics and is rich in large plagioclase phenocrysts, set in a matrix of phonolitic pumice and ash. The Roque Nublo monolith breccia may be of the same formation that caps the Mesa de Junquillo.